Positron Emission Particle Tracking Research Articles

A novel clustering approach to positron emission particle tracking

A novel approach to positron emission particle tracking is presented based on determining regions of space with high density of line of response crossing via clustering. The method is shown to be able to accurately track multiple particles in systems where the number of particles is unknown and in which particles can enter and leave the field of view of the scanning system. This method is explored in various environments and its parametric dependence is studied.

3D numerical simulation of a lab-scale pressurized dense fluidized bed focussing on the effect of the particle-particle restitution coefficient and particle–wall boundary conditions

3D numerical simulations of dense pressurized fluidized bed are presented. The numerical prediction of the mean vertical solid velocity are compared with experimental data obtained from Positron Emission Particle Tracking. The results show that in the core of the reactor the numerical simulations are in accordance with the experimental data. The time-averaged particle velocity field exhibits a large-scale toroidal (donut shape) circulation loop. Two families of boundary conditions for the solid phase are used: rough wall boundary conditions (Johnson and Jackson, 1987 and No-slip) and smooth wall boundary conditions (Sakiz and Simonin, 1999 and Free-slip). Rough wall boundary conditions may lead to larger values of bed height with flat smooth wall boundary conditions and are in better agreement with the experimental data in the near-wall region. No-slip or Johnson and Jackson׳s wall boundary conditions, with sufficiently large value of the specularity coefficient (ϕ≥0.1), lead to two counter rotating macroscopic toroidal loops whereas with smooth wall boundary conditions only one large macroscopic loop is observed. The effect of the particle-particle restitution coefficient on the dynamic behaviour of fluidized bed is analysed. Decreasing the restitution coefficient tends to increase the formation of bubbles and, consequently, to reduce the bed expansion.

Enhancing the activation of silicon carbide tracer particles for PEPT applications using gas-phase deposition of alumina at room temperature and atmospheric pressure

We have enhanced the radio-activation efficiency of SiC (silicon carbide) particles, which by nature have a poor affinity towards 18F ions, to be employed as tracers in studies using PEPT (Positron Emission Particle Tracking). The resulting SiC–Al2O3 core–shell structure shows a good labelling efficiency, comparable to γ-Al2O3 tracer particles, which are commonly used in PEPT. The coating of the SiC particles was carried at 27±3°C and 1 bar in a fluidized bed reactor, using trimethylaluminium and water as precursors, by a gas phase technique similar to atomic layer deposition. The thickness of the alumina films, which ranged from 5 to 500nm, was measured by elemental analysis and confirmed with FIB-TEM (focused ion beam – transmission electron microscope), obtaining consistent results from both techniques. By depositing such a thin film of alumina, properties that influence the hydrodynamic behaviour of the SiC particles, such as size, shape and density, are hardly altered, ensuring that the tracer particle shows the same flow behaviour as the other particles. The paper describes a general method to improve the activation efficiency of materials, which can be applied for the production of tracer particles for many other applications too.

Testing of a new dynamic Ergun equation for transport with positron emission particle tracking

The issue of formulating an Ergun‐like equation that applies to dynamic beds previously considered in Tupper et al. (2013) is revisited. Using new high quality positron emission particle tracking data, the volume and time averaged kinematic distributions of the granular (glass beads) and slurry (silica sand and water) mixture are computed. Incorporating these measured ingredients into the model then reveals that turbulence is not described by the usual effective viscosity, but nonetheless is negligible such that the new “Inertial Cell Equation” is first order in spatial derivatives. © 2015 American Institute of Chemical Engineers AIChE J, 62: 939–946, 2016

Observation of iron ore beneficiation within a spiral concentrator by positron emission particle tracking of large (Ø=1440 μm) and small (Ø=58 μm) hematite and quartz tracers

This paper presents the results of using positron emission particle tracking to record the trajectories of large (Ø≈1440μm) and small (Ø≈58μm) particles of hematite (S.G.=5.3) and quartz (S.G.=2.7) in a slurry of iron ore (20% solids w/w) flowing in a gravity spiral concentrator. The tracking was undertaken using modular positron emission particle tracking detectors (ECAT 951) assembled and calibrated for this purpose. The tracer particles used were generated by the direct activation of large particles in a cyclotron beam (3He, 35MeV), in the Positron Imaging Centre at the University of Birmingham. These larger particles were then broken and sized to isolate small active particles when required. The behaviour of the valuable and gangue particles in the first two turns of the spiral is presented. The formation and interaction of different bands of similar size/density particles is shown. For small particles, these bands are less defined and can be present at different radial positions on the trough for the same size and density material. Their formation and dispersion is influenced by the flow of the slurry. Part of the large dense hematite particles are shown concentrating toward the inside of the spiral trough while some others are shown remaining in the high flow-speed outer zone outside of a band of large quartz particles. The behaviour in the feed device and first 0.3 turn of the spiral is related to the later separation of some of the large particles and this highlights the effect of the feed radial distribution of the particles on the trough.

Speed analysis of quartz and hematite particles in a spiral concentrator by PEPT

Activated quartz and hematite tracers between 1180 and 1700μm in diameter were tracked in two sections of the first two turns of a spiral concentrator to ascertain information on valuable and gangue particle motion. The tracking took place while the tracer was flowing in a 20% solids by mass iron ore slurry. The direct activation of mineral particles, combined with the use of an adjustable height circular assembly of modular positron emission particle tracking detectors, has made this tracking possible. The tracer particle trajectories and speeds are presented in this paper. An early separation into two streams (concentrate or loss to tailings) can be seen for certain runs of the hematite tracers. Tracers speeds and radial position from the centre of the spiral give more details about the presence of the slurry film’s secondary flow in the middle zone of the spiral trough. Particle inward and outward migration speed in this secondary circulation is presented. The speed of this migration is approximately 0.12m/s inward and 0.15m/s outward for the set-up used.

Comparison of particle velocity measurement techniques in a fluidized bed operating in the square-nosed slugging flow regime

The novel “travelling fluidized bed” (TFB), operated under identical conditions, was deployed to compare alternate experimental measurement techniques for the investigation of solid motion in gas-fluidized beds operating in the square-nosed slugging regime. Measurements of particle velocity obtained by radioactive particle tracking (RPT — non-invasive at the Ecole Polytechnique de Montréal), positron emission particle tracking (PEPT — non-invasive at University of Birmingham), optical fibre probes (invasive at UBC) and borescopic high speed particle image velocimetry (invasive at PSRI) are compared for sand particles of mean diameter of 292μm. Significant differences between the time-average radial profiles of particle velocity are observed in many cases. The results provide valuable insights into the merits and challenges of advanced particle velocity measurement techniques.

Modelling the granular flow in a rotating drum by the Eulerian finite element method

This paper presents a numerical study on the flow behaviours of granular materials in a rotating drum based on the Eulerian-formulation finite element method (Eulerian FEM). The granular material is here assumed to be cohesionless and treated as a continuum medium described by the conventional Mohr–Coulomb elastoplastic model. It is shown that this Eulerian FEM approach can reproduce almost all the particle flow patterns that frequently happen in rotating drum, i.e. slipping, slumping, rolling, cascading, cataracting and centrifuging, provided that the parameters of rotational speed, coefficient of friction and internal friction angle are properly selected. On average, the predicted surface angles of a rotating bed are comparable to the experimental observations (Parker, D.J. et al., 1997. Positron emission particle tracking studies of spherical particle motion in rotating drums. Chem. Eng. Sci. 52, 2011–2022.), but the curvature of surface may be slightly over-estimated for drums comprised by big component particles. Linear distributions of particle velocity are seen along the mid-cord of the bed and agree well with the data published in the literature (Boateng, A.A., 1998. Boundary layer modelling of granular flow in the transverse plane of a partially filled rotating cylinder. Int. J. Multiphas. Flow 24, 499–521). The bulk stress inside the particle bed is also investigated, and its spatial distribution is demonstrated to depend on the rotating speed and other operational parameters.

Forced axial segregation in axially inhomogeneous rotating systems.

Controlling segregation is both a practical and a theoretical challenge. Using a novel drum design comprising concave and convex geometry, we explore, through the application of both discrete particle simulations and positron emission particle tracking, a means by which radial size segregation may be used to drive axial segregation, resulting in an order of magnitude increase in the rate of separation. The inhomogeneous drum geometry explored also allows the direction of axial segregation within a binary granular bed to be controlled, with a stable, two-band segregation pattern being reliably and reproducibly imposed on the bed for a variety of differing system parameters. This strong banding is observed to persist even in systems that are highly constrained in the axial direction, where such segregation would not normally occur. These findings, and the explanations provided of their underlying mechanisms, could lead to radical new designs for a broad range of particle processing applications but also may potentially prove useful for medical and microflow applications.

Laminar mixing in a SMX static mixer evaluated by positron emission particle tracking (PEPT) and magnetic resonance imaging (MRI)

Static mixers are implemented across many industries and are used for mixing, heating and reacting processes. This paper reports the combined use of positron emission particle tracking (PEPT) and magnetic resonance imaging (MRI) to assess mixing in the industrially important SMX static mixer geometry. The focus of this study was distributive mixing for a Newtonian fluid, glycerol, flowing in the laminar regime in the standard SMX static mixer. By implementing PEPT and MRI techniques, the work elucidated mixing indices that incorporate both local velocities and concentration fields within the structure of the mixer element at 0.5mm intervals over a length of nine 1.0L/D SMX elements. The experimental results are placed in context with previously published computational studies for this geometry.

Three-dimensional dynamic imaging of sand particles under wheel via gamma-ray camera system

In recent years, attempts have been made to deploy robots for use in various activities such as planetary exploration, post-tsunami seashore reconnaissance, and volcano investigations. These robots may have to move on soft terrain. The movement of sand or soil particles under the wheels or tracks greatly affects the robot’s ability to maneuver. There is a simple but difficult problem with measuring particle movement: the sand and soil particles beneath the surface are not visible. Only 2D visualization techniques that take a surface picture of the ground or use transparent boards are available. A nuclear 3D imaging technique called positron emission particle tracking (PEPT) was developed at the University of Birmingham for this purpose. PEPT detects pairs of gamma rays emitted by a positron-emitting radionuclide of a tracer particle, which produces an image of the tracer. Thus, the overarching goal of this study was to explore the 3D terramechanics between terrain particles and a wheel or track using PEPT. As an initial step, this paper introduces an imaging technique for standard sand under a rotating wheel using PEPT and presents some images of sand particles under various conditions. Absolute displacements along the longitudinal, vertical, and lateral axes are presented.

The use of positron emission particle tracking (PEPT) to study milling of roll-compacted microcystalline cellulose ribbons

Milling is a critical process for controlling the properties of the granules produced by roll compaction. In the current study, the positron emission particle tracking (PEPT) technique was used to examine the milling kinematics of roll-compacted ribbons at various milling speeds. Microcrystalline cellulose (MCC, Avicel PH-102) was used as the model feed material and a radioactive particle (tracer) was mixed with the MCC powder and roll-compacted to form sample ribbons. They were then milled using an oscillating mill at various speeds and the kinematics of the ribbons (trajectory, velocity, and occupancy) were quantitatively determined using PEPT. A close examination of the PEPT data reveals that, for milling MCC PH-102 ribbons using the oscillating mill considered in this study, the milling speed plays an important role: at low values, the milling process is dominated by cooperative motion of the ribbons with the blade (i.e. the speeds of the ribbons and the blade are similar, and the ribbons move along with the blade) and the ribbons are milled primarily by abrasion; as the speed increases the ribbons undergo more random motion involving collisions that results in an increase in ribbon breakage and hence an increase in the milling efficiency. It is shown that the PEPT technique is a useful technique for examining milling kinematics of roll-compacted ribbons.

Maximizing energy transfer in vibrofluidized granular systems.

Using discrete particle simulations validated by experimental data acquired using the positron emission particle tracking technique, we study the efficiency of energy transfer from a vibrating wall to a system of discrete, macroscopic particles. We demonstrate that even for a fixed input energy from the wall, energy conveyed to the granular system under excitation may vary significantly dependent on the frequency and amplitude of the driving oscillations. We investigate the manner in which the efficiency with which energy is transferred to the system depends on the system variables and determine the key control parameters governing the optimization of this energy transfer. A mechanism capable of explaining our results is proposed, and the implications of our findings in the research field of granular dynamics as well as their possible utilization in industrial applications are discussed.

Residence time distributions of different size particles in the spray zone of a Wurster fluid bed studied using DEM-CFD

Particle cycle and residence time distributions in different regions, particularly in the spray zone, play an important role in fluid bed coating. In this study, a DEM-CFD (discrete element method, computational fluid dynamics) model is employed to determine particle cycle and residence time distributions in a laboratory-scale Wurster fluid bed coater. The calculations show good agreement with data obtained using the positron emission particle tracking (PEPT) technique. The DEM-CFD simulations of different size particles show that large particles spend a longer time in the spray zone and in the Wurster tube than small particles. In addition, large particles are found on average to move closer to the spray nozzle than small particles, which implies that the large particles could shield small particles from the spray droplets. Both of these effects suggest that large particles receive a greater amount of coating solution per unit area per cycle than small particles. However, the simulations in combination with the PEPT experiments show that this is partly compensated for by a longer cycle time for large particles. Large particles thus receive more coating per unit area per pass through the spray zone, but also travel through the spray zone less frequently than small particles.

Validation of hydrodynamic and microbial inactivation models for UV-C treatment of milk in a swirl-tube ‘SurePure Turbulator™’

The Positron Emission Particle Tracking (PEPT) flow visualisation technique has been applied to determine the hydrodynamic performance of a full-scale transparent model of a SurePure Turbulator™ used for the microbial treatment of turbid dairy fluids using UV-C radiation. The effect of flow rate upon the refreshment of fluid at the surface of the UV source has been investigated using two model fluids each possessing the same viscosities as milk and cream respectively. The amount of surface refreshment is modelled as a time density function close the surface of UV-C source and incorporated into an existing first order microbial inactivation model and a Weibull distribution model. Fitting to experimental data obtained for the inactivation of selected milk pathogens using the Turbulator™ have demonstrated the superiority of the Weibull model. These models enable a more precise estimation of UV-C energy requirement for the inactivation of the milk borne pathogenic organisms to be made since the amount of surface refreshment affords a significant performance enhancement.

Positron emission particle tracking in fluidized beds with secondary gas injection

We apply positron emission particle tracking (PEPT) to a gas–solid fluidized bed with injection of a secondary gas through a centrally arranged nozzle and present a method to compute stationary fluid-dynamic characteristics of the system from the trajectories of a test particle. In order to evaluate this non-invasive method we compare the field of density obtained by PEPT with the density obtained by a traditional, well-established and approved, yet invasive, measurement technique to find good agreement. Besides the penetration depth of the jet region and the opening angle of the jet which are inferred from the density field, we use PEPT to measure quantities whose measurement using traditional methods is rather sophisticated, including the residence time of particles in the jet region and the suspended phase, the coefficients of axial and radial dispersion and the material flux across the jet boundaries. We conclude that PEPT is a reliable and at the same time versatile technique to measure stationary fluid-dynamic properties of dynamical particle systems at spatial resolution only limited by the duration of the measurement.

Lagrangian particle tracking in mechanically agitated polydisperse suspensions: Multi-component hydrodynamics and spatial distribution

We investigated the local hydrodynamics and phase distribution of complex polydisperse suspensions ‘just-suspended’ by a down-pumping pitched blade impeller in a mechanically agitated vessel. The solid–liquid mixtures consisted of five size fractions of coarse glass particles in water, and the total concentration was varied within the range 2.4–23.6vol% (5–40wt%). The whole flow field and spatial distribution of the liquid phase and of each particle size fraction were resolved using the Lagrangian technique of positron emission particle tracking (PEPT). For the first time, it has been possible to conduct such a detailed ‘pointwise’ examination within an opaque polydisperse suspension of this type and complexity. For each component of the two-phase flow, results are presented in the form of maps of local velocity, solid concentration and time-averaged slip velocity, as well as plots of spatial suspension uniformity index. The accuracy of measurements is verified through six-component mass balance and mass continuity calculations.

Investigation of particle velocity in FCC gas-fluidized beds based on different measurement techniques

The novel traveling fluidization column, designed and built to assure identical operating conditions, was deployed to compare alternate experimental measurement techniques for hydrodynamic characterization of gas-fluidized beds. This paper compares measurements of particle velocity obtained by radioactive particle tracking (RPT—non-invasive at the Ecole Polytechnique), positron emission particle tracking (PEPT—non-invasive at University of Birmingham), optical fibre probes (invasive at UBC) and borescopic high speed particle image velocimetry (invasive at PSRI) carried out with FCC particles of mean diameter 107μm. All of the techniques provided similar trends with respect to time-average particle velocity profiles, but significant differences were observed in some cases. Analysis of the results, focusing on the physical principles of each measurement technique, provides valuable insights into the reasons for the observed discrepancies. The results also add to a unique hydrodynamic database for validation of CFD and other mechanistic models.

Investigation of granular impact using positron emission particle tracking

We present results from an experimental study of granular impact using a combination of high-speed video and positron emission particle tracking (PEPT). The PEPT technique exploits the annihilation of photons from positron decay to determine the position of tracer particles either inside a small granular bed or attached to the object which impacts the bed. We use dense spheres as impactors and the granular beds are comprised of glass beads which are fluidised to achieve a range of different initial packing states. For the first time, we have simultaneously investigated both the trajectory of the sphere, the motion of particles in a 3-D granular bed and particles which jump into the resultant jet, which arises from the collapse of the cavity formed by the impacting sphere.

Analysis of the mixing of solid particles in a plowshare mixer via discrete element method (DEM)

Mixing of powders is an important unit operation in many industries. Discrete element method (DEM) was employed in this study to characterize the mixing of the solid particles in a plowshare mixer. To validate the model, the simulation results were compared to the Positron Emission Particle Tracking (PEPT) data reported in the literature by Laurent and Cleary for a plowshare mixer. The parameters used for validation were the mean square of axial and radial displacements of particles. The simulation results were in good agreement with the experimental data. The validated DEM was then utilized to calculate the mixing index as a function of the initial loading, rotational speed of impeller, fill level, and particle size for a six-blade plowshare mixer. In order to employ the Lacey index for the assessment of the degree of mixing, the mixing domain was discretized using a mesh system and the variance of the mixture was calculated. Moreover, the mixing time, which is the time required to reach a homogeneous mixture, was presented as a function of the operating conditions.